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Wednesday, September 16, 2015

Drama at the Canyon Mouth

In northwest Wyoming, roughly 30 miles northwest of Powell, the Clarks Fork of the Yellowstone River leaves the Beartooth Mountains with great drama—by way of a canyon mouth so geologically spectacular that it’s included in field trips for everyone from high school teachers to professionals. Unfortunately, the rocks are a bit hard to explain. But the scenery’s great, even on cloudy days.

Looking downstream at south side of canyon mouth and basin beyond.

North side of canyon mouth.

Steeply-tilted Paleozoic sedimentary rocks rise abruptly from the lowlands, and are in contact with Precambrian rocks to the west. Between them is a 2-billion-year gap in the rock record, a Great Unconformity.

Arrow points to Great Unconformity between brownish Cambrian Flathead sandstone (500 million years old) and Archean metamorphic rocks (2.7 billion years) on north side of canyon. See the magpie? (lower right)

Most mountain ranges in Wyoming were uplifted roughly 70-40 million years ago during the Laramide orogeny, the event that created the Rocky Mountains. They have a distinctive style: broad blocks of crust shoved up along reverse faults over younger rocks.

Laramide-style uplift; from Regional Geomorphology, University of Wyoming, 1984.

This diagram shows a typical Laramide range. On the east, Precambrian (ancient) igneous and metamorphic rocks (brown) have been shoved up and over younger sedimentary rocks along high-angle reverse faults; Precambrian rocks make up the east flank and crest. On the opposite side, the rocks were folded rather than faulted, and there are outcrops of tilted sedimentary strata (in this case eroded to form hogbacks and strike valleys).

At the canyon mouth, the Clarks Fork cuts through steeply-tilted sedimentary strata. Here the Beartooth front is steep and abrupt, and decorated with contorted sculpted rock.

Can you imagine rocks being folded like this?

In less than a quarter mile, the tilted strata start to flatten out.

Immediately west, the strata are nearly horizontal—note limestone bed on skyline. Further west sedimentary strata are gone, having been eroded off the crest of the range.

When I first saw the dramatic exposures of eroded sedimentary rocks on the Beartooth front, I presumed I was looking at a steeply-tilted, fault-free flank. Wrong. Here the Beartooth uplift was shoved east over younger rocks, via high-angle reverse faults (like the east side of the "typical" Laramide uplift above). So what’s with the sedimentary strata?

The story is complicated and controversial. Popular geology guides wisely keep explanations short. In his great guide to geology east of Yellowstone Park, Bob Carson quickly passes by deformed sedimentary strata, and focuses on Precambrian rocks, the Great Unconformity and glacial features. In Roadside Geology of Wyoming, Lageson and Spearing include a cross-section showing the Beartooth reverse fault cutting through Precambrian and sedimentary rocks (basis for diagram above). Sedimentary strata are draped over faulted Precambrian rock. But discussion is brief: “Beds of Paleozoic limestone were folded over the edge of the Beartooth uplift, then eroded”

I defer to experts for further discussion (see Sources below). Here’s some of what they’re thinking:

In the “spectacular fold at the mouth of the canyon” sedimentary strata are draped over a faulted basement (Precambrian) block. This structure is slightly older than the Beartooth uplift and therefore was deformed by those faults (various authors; summarized in Heasler et al. 1996; PDF).

Laramide models used to incorporate vertical uplift. But with seismic and drill-hole data, it has fallen out of favor, replaced by uplift via high-angle reverse faults, accommodating crustal shortening. However in situations where faults are very steep (e.g. Beartooth fault at Clarks Fork Canyon), the vertical uplift model may be valid, and sedimentary strata may well be draped over faulted basement rock (Brown 1995).

O’Connell (1996) argued for multiple stages of faulting, and invoked zones of preexisting weakness in basement rocks. He argued against drape-folding.

The southeast Beartooth front is in a complex transition zone involving the western Bighorn Basin, with its own faults and Laramide history. Thus the front has been affected by multiple episodes of deformation (Neely 2006; PDF).

“We gather at the base of the crag to study the rocks. The instructor assigns them to type and age, and proclaims the cause of their deformation—all the while waving his arms in the air while we frantically write everything down in our field books. Later, we read that others think differently.” (anonymous geology student ca. 1985)

2 comments:

Definitely some drama there! I find the transition zones--all types of transition zones: geological, ecological, climate, etc.--to be particularly fascinating. Thanks so much for explaining this important location and its events with us.